1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing of the semiconductor device. In addition, the present invention relates to a display device provided with the semiconductor device of the present invention, and an electronic device provided with the display device.
It is to be noted that, in this specification, a semiconductor device indicates all devices which can function by utilizing semiconductor characteristics.
2. Description of the Related Art
In recent years, a technique of making a thin film transistor (TFT) by using a thin semiconductor film (a thickness is approximately several to several hundreds nm) formed over a substrate having an insulating surface has been attracting attention. The transistor is widely applied to electronic devices such as an IC, an electro-optical device, and a light-emitting display device, and development thereof as a switching element for an image display device has been particularly expected.
In addition, an active matrix display device (a light-emitting display device or a liquid crystal display device), in which a switching element formed using a transistor is provided in each of display pixels arranged in a matrix, has been actively developed. In order to improve image quality of a display device, a switching element capable of operating with high-speed is required; however, there is a limitation to develop such a switching element with a transistor using an amorphous semiconductor film. Therefore, a transistor using a crystalline semiconductor film having higher field-effect mobility is used, and it is necessary to form a crystalline semiconductor film with favorable quality.
In Patent Document 1 (Japanese Published Patent Application No. 2003-68642), there is disclosed a technique in which a catalyst element which promotes crystallization such as nickel is added, an amorphous semiconductor film is crystallized by heat treatment, an obtained crystalline semiconductor film is irradiated with laser light, and crystallinity of the crystalline semiconductor film is enhanced. Further, there is disclosed a technique in which, after crystallization, the catalyst element left in the crystalline semiconductor film is gettered to an amorphous semiconductor film containing phosphorus or a rare gas element.
However, in the technique disclosed in Patent Document 1, a measure against pin holes or unevenness generated in the crystalline semiconductor film after gettering is not sufficient.
When an amorphous semiconductor film is crystallized by adding a metal element such as nickel and carrying out heat treatment, an oxide film is formed over a surface of a crystalline semiconductor film that is formed. In a case where the oxide film is removed by using a hydrofluoric acid based solution, steps of hydrofluoric acid treatment, cleaning with pure water, and drying are sequentially carried out. However, when the steps from hydrofluoric acid treatment to drying are sequentially carried out as described above, the exposed semiconductor film, oxygen in atmospheric air, and H2O molecules in pure water react with one another when removing the oxide film with hydrofluoric acid and cleaning with pure water. Therefore, a reaction product is generated over the crystalline semiconductor film. In general, this reaction product is referred to as a watermark.
In a case of manufacturing a semiconductor device, a metal element such as nickel has a function of promoting crystallization. However, when the metal element is left in a semiconductor film after crystallization, characteristics of the semiconductor device are adversely affected; therefore, the metal element is required to be removed. As one method for removing the metal element, as described in Patent Document 1, there is known a technique in which an oxide film and a semiconductor film containing a rare gas element are stacked over a crystalline semiconductor film, and a metal element is gettered from the crystalline semiconductor film to the semiconductor film containing a rare gas element by heat treatment. It is to be noted that, in gettering, a metal element contained in a region to be gettered (a semiconductor film after crystallization) is discharged by thermal energy and moves to a gettering region (a semiconductor film containing a rare gas element) by diffusion.
After reducing or removing the metal element from the crystalline semiconductor film, the oxide film and the semiconductor film containing a rare gas element over the crystalline semiconductor film are removed. Specifically, the semiconductor film containing a rare gas element is removed by etching selectively by using an alkaline solution such as tetramethylammonium hydroxide (TMAH) or choline (an aqueous solution of 2-hydroxyethyltrimethylammonium hydroxide). At this time, the reaction product, which is generated in removing the oxide film after the crystallization, is also etched, and pin holes are generated in the crystalline semiconductor film.
In addition, there are various factors to cause pin holes. When pin holes are generated in a crystalline semiconductor film, characteristics of a semiconductor device are adversely affected; for example, coverage of a gate insulating film that is formed later becomes worse and a defect is caused, and the like. Accordingly, it is a highly important object to reduce pin holes in a crystalline semiconductor film that is finally obtained and planarize the crystalline semiconductor film.